The natural forgetting part caught my eye. Is the decay time based, driven by retrieval frequency, or scored by the reranker?

In short, it's both. I borrowed the idea from Bjork's New Theory of Disuse: each memory has two strengths. One decays with time, the other grows with successful recall and slows that decay. The reranker is more of a final-stage relevance check than part of the forgetting itself.